Spectroscopy is a tool for remote, non-destructive, non-invasive, or minimally invasive analysis of elemental content or molecular composition of samples. Spectral content of the electromagnetic radiation (EM radiation) that is emitted by the sample under specific conditions, or produced by external source and modified as a result of interaction with the sample may be analyzed using spectroscopy.
Applications of spectroscopy include chemical composition determination, sample identification and differentiation and may be used in applications such as food processing, food sorting, petrochemical analysis; chemical composition identification of ore, minerals, pharmaceutical agents; differentiation between healthy and disease affected tissues; analysis of chemical composition of solids, powders, fluids, gases, and mixtures of states of matter; identification of chemical composition of fume exhausts, gas clouds, plasma clouds or mist clouds, detection of impurities, detection of foreign bodies, detection of biological abnormalities, identification of electromagnetic discharge, sorting of materials for industrial purposes, sorting of materials prior to processing of said materials, identification of materials prior to processing, plant identification in agriculture, environmental testing, identification of dangerous substances, identification of aromatic substances.
Over a range of wavelengths from about 175 nm to about 10,000 nm (wavelengths below 175 nm and above 10,000 nm may also be included), optical methods may be used to perform analysis of spectral composition of electromagnetic radiation. This part of the electromagnetic spectrum (from about 175 nm to about 10,000 nm) may be referred to as the optical spectrum, or “light”. Methods used to analyze the spectral composition of EM radiation in an optical spectral range spatially separate different components of the spectra (i.e. spatially dispersive methods) may use dispersive optical components like prisms, periodical light-scattering structures, nanostructures, micro-mirror arrays, MEMS (microelectromechanical system) structures, diffraction gratings, or a combination thereof, as would be known to one of skill in the art. The capability of instruments using dispersive components can be dramatically expanded by using two dimensional detector arrays.
Reflecting diffraction gratings, ruled, etched or holographic, or etched, holographic or volume transmission gratings may be used for the dispersion of light. Regardless of the structure of the spectroscopic system using optical diffractive components, the general light propagation path, including conversion of information contained in the spectrum of electromagnetic radiation to an electrical signal, and treatment of this electrical signal, is similar for most if not all instruments.
Dispersive spectroscopic systems generally include a source of EM radiation, for example an external light source. Radiation from the EM source is collected using a source light collector (SLC), and the collected light is transformed into a spatial light distribution that is optimal for sample illumination. The spatial light distribution or structured illumination light, is produced using an illumination light structuring component (ILSC) and interacts with a sample to produce light that contains information about the composition of the sample. Following interaction with the sample, the information containing light may then be analyzed spectrally. The information containing light obtained from the sample is collected with sample light collector (SaLC) and delivered to spectrometer light structuring component (SLSC), which produces a spatial light distribution that is optimal for the performance of a light dispersing engine (LDE). The light dispersing engine accepts the light delivered by SLSC, and restructures the spatial intensity light distribution to obtain an efficient detection and separation of different spectral components of the light with photo detectors (PD). The photo detectors convert the optical signal into an electrical digital signal using an electrical signal converter (ESC) while preserving information extracted with spectroscopic system. The electrical digital signal is transferred to a data pre-processing unit (DPPU), where data is prepared for analysis in a data analyzer (DA) so that desired chemical information may be extracted from spectroscopically collected physical data.
Sunlight may be used as a natural source of light for many spectrometric systems, especially those which are used to analyze optical properties of the earth or any object on the earth's surface. For many applications; however, the light is produced by some artificial or synthetic means such as incandescent, halogen, metal-halogen, high pressure vapor, high pressure gas, low pressure gas and vapors, light emitting diodes (LEDs), lasers and others as would be known to one of skill in the art, can be used as a single light source or in combination to produce light with required optical characteristics. For example, light from plurality of light sources can be combined to produce light with broadband spectrum to fit requirements of the used spectral analyzer whose technical characteristics might be optimized to extract maximum information from the measured samples. Both spatial and spectral structuring of light is beneficial to optimize information to be obtained during sample analysis. By combining multiple sources comprising different spectral behaviors together, light may be structured spectrally so that it is optimized to match the properties of the sample, the spectroscopic system, or the detector.
The spectral and spatial structuring of light from one or plurality of light sources through the illumination light structuring component, may include various optical stigmatic or (and) astigmatic optical components such as lenses and mirrors transforming the collected light into a convergent, divergent or collimated light beams with different characteristics for two orthogonal propagation planes, light guiding rods, pipes and optical fibers, diffusers to scatter light for scattered light spectroscopy, polarizers to select preferred polarization of light, filters to filter out an undesired part of light, neutral density filters to adjust signal level to that acceptable by light measuring component, as well as light combiners such as beam splitters, polarization beam splitters, fiber optic combiners, prisms, dichroic mirrors, diffractive elements and others for enhancement of this light with additional band light produced by suitable light sources (various spectral lamps), or with narrow bands of light produced by LED and lasers (U.S. Pat. No. 6,765,669; to Pawluczyk, which is incorporated herein by reference, shows two light sources being combined, for example). Light produced in such way is used as a tool to extract required spectral information from the tested samples and as a carrier to deliver that information to spectral light analyzer. The light obtained from a single light source or from combined light sources may need to be structured in different ways depending on the application. For example:                to focus light delivered by a light source, for example a laser, into a point on the sample as for example with confocal microscopes, Raman confocal microscope, Laser Induced Breakdown Spectroscopy (LIBS); to create a high intensity light distribution along the straight or curved line, or plurality of lines, along the sample, for spectral imaging optical instrument;        to create a high intensity light distribution of high intensity point or plurality of points along the sample in a pre-determined manner, for spectral imaging optical instrument;        to collimate the light, to illuminate transparent samples in cuvettes with plane, parallel transparent walls;        to produce diffused illumination for light scattering samples for diffused light spectroscopy.        
Other spatial and spectral distributions optimized for the purpose of collecting optimal information from the sample may also be used. For example, in some cases it may be important to know the initial spectral composition of light delivered to the sample from an external source or plurality of sources. This may be achieved by dividing the light delivered to the sample into two light paths from which one path, for example, the more powerful path is used to illuminate the sample, while the second path is directed to separate detector to monitor the variability of light intensity produced by the light source(s). Alternatively, the intensity and spectral content of the used light may be monitored by periodically switching the light path from illumination of samples to directly coupling into the light analyzing unit through the spectrometer light structuring component (SLSC).
Upon interaction with a sample, spectrally and spatially structured light is modified as photons are absorbed, reflected, scattered, transmitted, spectrally shifted or deflected by the atoms, molecules and structures in the sample. The sample itself may also become a source of light in this process, since electromagnetic radiation of various wavelength may be emitted by the sample when for example the sample is heated to high temperatures, the sample is evaporated, electrical currents are conducted through the sample, plasma is produced using electrical current (plasma excitation), as well as other mechanisms, including luminescence, fluorescence, electroluminescence, Raman, temperature related emission, vapor or plasma emission, impact fluorescence and other light effects produced by different physical processes.
Once light interacts with, or is produced by the sample, this sample-affected light may then be processed with the subsequent parts of the dispersive spectral system. When an Illumination Light Structuring Component (ILSC) is used, there may be a need to collect the sample-affected light and restructure it for the delivery to the light dispersing engine. In this case the sample-affected light is collected with the sample light collector (SaLC) and delivered to the spectrometer light structuring component (SLSC) which produces a distribution of spatial light that is optimal for the light dispersing engine (LDE). This step may require transformation or restructuring of the collected light so that the intensity of light is spatially structured as one, or a plurality of lines projected on straight slit or slits of the LDE. These lines may also comprise a different shape, for example, to compensate for astigmatism produced by the LDE.
Restructuring of the collected light can be achieved for example, using fiber optic bundles which have a different spatial distribution of fibers at both ends of fiber optic bundle. For example, at one end, fibers can be tightly compressed into a circle, while at another end those fibers are rearranged into one or a plurality of lines to fit slit of the LDE. Alternatively, various optical components like lenses, light-conducting roads, optical fibers, light attenuators, spectral light-shaping filters, polarizers, and other optical components can be used to modify various optical properties of analyzed light, to achieve a desired performance of the spectroscopic system. Light collected with the SaLC may form a circular spot of a diameter much larger than the width of entrance of slit of the LDE, this light may be transformed by SLSC into the form of a straight line. For example, the transformation of light may involve optical devices such as cylindrical lenses, non-spherical or spherical mirrors, prisms and lenses, or fiber optics which collect light from a circular spot at one end and arrange the light in a line at the end which couples to the LDE. Another example of light restructuring involves splitting the circular spot of light in two half circles placed one over another producing an elongated light source, thereby illuminating an elongate slit (US 2011/0299075, to Jeffrey T. MEADE et al.).
Detection of dispersed light produced by the LDE may involve the use of scanning systems, for example monochromators, which extract, step by step, narrow spectral bands of light received from the LDE following sample illumination with narrow light bands. Alternatively, Czerney-Turner or Seya-Namioka spectrometers may be used. These spectrometers use a broad band spectrum of light to illuminate the sample and extract narrow bands of sample affected light for measurement. This approach presents the amplitude of a signal for a single narrow spectral band of light that is collected by a single photo detector (PD) at a given point in time. Multiple measurements for adjacent bands can be collected and processed over time for the entire wavelength range of interest. These systems are inefficient in light collection since resolution dependent, narrow bands are collected for analysis, while other bands are ignored. Therefore efficiency of these instruments deteriorates with resolution, thereby increasing measurement time and increasing the amount of unused light. The unused light contributes to increased background scatter light and further deteriorates the quality of the obtained results. Traditional LDE present spectral information in the form of a one dimensional array of data, which shows the amplitude of signal for a given wavelength, the wavelength shift, or the frequency of light.
Another method of generating data using an LDE is to use dispersive optics to spatially separate narrow bands of wavelengths of light and direct these bands to spatially differentiated points, where differentiated detector elements are located. The measurements are then performed with a linear array of photo detectors such as a line-scan camera, a plurality of adjacent photodiodes, an array of adjacent CCD (charge coupled devices), CMOS (complementary metal oxide semiconductor), an array of adjacent photomultipliers, an array of photodiodes, or other detectors as would be known to one of skill. This type of light detection is more efficient than the single photo detector method, yet the final spectral intensity distribution is presented in a one dimensional array.
The Light Dispersing Engines described above do not differentiate information collected at different spatial points along the spectrometer entrance. Furthermore, some instruments, such as Seya-Namioka spectrometers compress narrow spectral bands of light emerging from a slit to fall on a single detector without distinguishing the information content coming from different points of that slit. These spectrometers do not present information variability contained in a spatial distribution of spectrally dispersed light. However, the spatial distribution of spectrally dispersed light contains additional information about the sample under observation and the lack of differentiation of spatial distribution diminishes the utility of traditional one dimensional LDEs.
Two dimensional detectors are typically used in a manner similar to one dimensional detectors. Binning is used to produce a one dimensional array of information collected from a larger area, thus improving signal strength (“binning mode” where an electronic readout of the 2D detector is compressed to contain information in a one dimensional vector).
More complex spectroscopic data collection and spatial differentiation of spectrally dispersed light requires differentiation of both spatially and spectrally distributed light. Such systems include light dispersing systems which use an entrance slit or slits, illuminated by electromagnetic radiation delivered from a plurality of points, containing both spatial and spectral information. These advanced light dispersing systems may be used to analyze spectral information from plurality of points simultaneously and require specialized optical designs, such as aberration corrected optical design for imaging reflective gratings, optics, or the use of aberration corrected imaging lens based optics and transmission gratings (see for example U.S. Pat. No. 7,315,371; U.S. Pat. No. 6,266,140; US2005162646A1).
The light exiting currently available light dispersing engines is presented as either a one dimensional array of intensities varying for different points along the light spectrum, or a two dimensional array of intensities where one direction provides location along the light spectrum, and the other direction of the array provides information about the spatial location from which the signal was collected. Usually, these two signals cannot be considered as independent, as presence of geometric distortions shifts the spectral array for different positions along the spectral direction. The signal magnitude of each element or “pixel” of the two dimensional matrix typically corresponds to the number of photons which have been detected on the given element of the photo detector. This data is typically transformed into a digital signal by the electric signal converter which converts the optical signal (photons) into electron current. The electron current is digitized and delivered to a data pre-processing unit (DPPU) for initial processing and data optimization and finally to a data analyzer (DA) and, or display. These photon counts are typically divided into spectral bands, and typically photons of a specific narrow band of light contribute to a pixel. Photons hitting a single pixel may also include photons which have not been sorted, but are scattered or noise photons.
At each step of this process, including capturing photons, converting photons into current, and digitization, statistical processes are involved and noise elements are introduced in the analysis. Furthermore, when conversion of photons into electrons occurs, other electromagnetic radiation such as cosmic rays may also influence the conversion, in addition to scattered photons being registered on a pixel. The electronic noise of the photo detector also contributes to the overall noise. Noise of photo detector arrays is typically classified as spatial and time dependent. Furthermore, most electronic detector arrays are significantly impacted by sensor temperature. The equation:
  SNR  =                    PQ        e            ⁢      t                                            (                          P              +              B                        )                    ⁢                      Q            e                    ⁢          t                +        Dt        +                  N          r          2                    
Describes the noise level of each individual pixel, where:                SNR is the Signal to Noise Ratio of the detector,        P is the incident photon flux,        Qe is the Quantum efficiency of the electronic detector,        t is the integration time (in seconds),        B is background photon flux due to scatter of the entire optical system,        D is the dark current value (electrons/pixel/second), and        Nr represents the read noise (electrons rms/pixel),governs the Signal to Noise Ratio or the inherent noise level of the photo detector. Dark current is dependent on temperature, and tends to increase as temperature of the detector increases. Many methods have been developed for reduction of noise in both imaging application and spectroscopic applications. The typical approaches of averaging, binning or taking multiple measurements is flawed, as the noise profile of only the photo detector is taken into account, and the systemic noise sources of spectroscopic system are not all considered.        
Most methods to process signals and reduce noise are directed to image creation, and are not optimized for spectroscopic analysis. However, many modern spectrometers utilize the photo detector signal to collect spatial and spectral information within a two-dimensional matrix with both dimensions of a matrix corresponding to the coding of spatial geometry, rather than form an image. Methods that increase the information content of the signal, and increase the signal to noise ratio are desired.